The short version
- St John's wort activates the pregnane X receptor and induces CYP3A4 and P-glycoprotein, with documented pharmacokinetic effects on cyclosporine, tacrolimus, digoxin, indinavir, warfarin, alprazolam, simvastatin and oral contraceptives.
- In a systematic review of four studies, St John's wort taken with combined oral contraceptives increased breakthrough bleeding in three, reduced hormone exposure in three, and produced probable ovulation in one, raising concern about contraceptive efficacy.
- More than 85 drugs are known or predicted to interact with grapefruit, and the number capable of causing serious adverse effects rose from 17 to 43 between 2008 and 2012 through irreversible inhibition of intestinal CYP3A4.
- In 66 patients on long-term warfarin, higher dietary vitamin K intake was associated with more stable anticoagulation, with an INR coefficient of variation of 19.2% above 195.7 micrograms a day versus 25.5% below 126.5 micrograms a day.
- In 1,516 cardiac surgery patients, perioperative fish oil did not increase major bleeding (odds ratio 0.81, 95% CI 0.53 to 1.24) and significantly reduced units of blood transfused.
This is not an exhaustive list and no article should pretend to be one. It is the short set of interactions that are well documented, common enough to matter, and repeatedly missed — plus the correct move at the end, which is to take your actual bottles to a pharmacist.
One framing note first. "Natural" has no bearing on this. An interaction happens because a compound changes how an enzyme or a transporter behaves, and plants are extremely good at that.
St John's wort, and why it belongs in a category of its own
St John's wort is a potent activator of the pregnane X receptor, which induces cytochrome P450 enzymes — most importantly CYP3A4 — and the P-glycoprotein transporter. In practice that means it speeds up the clearance of a long list of drugs. A 2020 review documents altered pharmacokinetics for cyclosporine, tacrolimus, digoxin, indinavir, warfarin, alprazolam, simvastatin and oral contraceptives, and notes that the degree of CYP3A4 induction correlates with the hyperforin content of the preparation. In 2000, an interaction with cyclosporine caused acute rejection in two heart transplant patients.
The contraception case deserves its own paragraph because it affects a lot of people who would never think of themselves as being on a serious medication. A CDC-authored systematic review found four studies comparing combined oral contraceptives alone against the same pills taken with St John's wort. Three found an increased risk of breakthrough bleeding. Three found pharmacokinetic changes indicating reduced hormone exposure. One found increased follicular growth and probable ovulation. The authors' conclusion was that this raises concern for decreased contraceptive efficacy.
If you take hormonal contraception and you are considering St John's wort for mood, that is a conversation to have before you start, not after.
Grapefruit, which is a food but behaves like a drug
The furanocoumarins in grapefruit are metabolised by CYP3A4 into intermediates that bond covalently to the enzyme's active site and inactivate it irreversibly. Intestinal CYP3A4 stays impaired until the body synthesises new enzyme, which is why timing your grapefruit away from your tablet does not solve it. The effect goes the opposite way to St John's wort: drug levels rise rather than fall.
More than 85 drugs are known or predicted to interact. Between 2008 and 2012, the number with potential to interact and cause serious adverse effects rose from 17 to 43. Statins are the household example, but the list runs well beyond them, and it is worth checking rather than assuming.
Vitamin K and warfarin, which is about consistency rather than avoidance
Warfarin works by antagonising vitamin K, so vitamin K intake moves the INR. The instinct is to avoid it, and that is the wrong lesson. In 66 patients on long-term warfarin, higher dietary vitamin K intake was associated with more stable anticoagulation: the group above 195.7 micrograms a day had a lower coefficient of variation of INR than the group below 126.5 micrograms a day (19.2% versus 25.5%, P < 0.05).
What destabilises an INR is change. A K2 supplement is a step change in vitamin K intake, and the K2 with D3 combination products are an easy way to make one without noticing. If you take warfarin, no vitamin K supplement should start or stop without your anticoagulation clinic knowing.
Minerals competing with each other
This is the quiet one, because both items are things you were told to take. Calcium reduces non-heme iron absorption; in human absorption studies, 300 to 600mg of calcium cut it substantially, and the effect appeared with calcium delivered as milk or cheese as well as from a tablet. Zinc competes with copper and with iron in the same way.
The fix is scheduling, not abandonment. Iron away from calcium supplements and away from a dairy-heavy meal; zinc separated from a high-dose iron supplement. Anyone taking iron for a documented low ferritin should get this right, because it can be the difference between the course working and not.
The one you have probably been told wrongly
Fish oil before surgery. The standard instruction is to stop a week or two in advance because of bleeding risk. A trial of 1,516 cardiac surgery patients randomised to perioperative fish oil (8 to 10g of EPA plus DHA for 2 to 5 days before surgery, then 2g a day after) or placebo tested it directly. Major bleeding occurred in 6.1% overall, and the odds ratio in the fish oil group was 0.81 (95% CI 0.53 to 1.24) — not higher. Total units of blood transfused were significantly lower with fish oil (1.61 versus 1.92, P < 0.001), and higher achieved omega-3 levels were associated with lower bleeding risk.
The authors said the finding supports reconsidering the recommendation to stop fish oil before procedures. Follow your surgeon's instruction regardless — they are managing your case, not a trial average — but it is worth knowing this particular caution is not as well founded as its confidence suggests.
What we'd actually tell you
Put every bottle in a bag, including the ones you think are trivial, and show a pharmacist. It is free, it takes ten minutes, and they will find the ones an article cannot: the zinc that blunts your doxycycline, the timing problem with your thyroid tablet, the herb in a sleep blend nobody read the back of.
And if you take a prescription medication daily, treat any new supplement as a change to your regimen rather than an addition to your groceries.
Good questions
Can St John's wort stop my birth control from working?
It can reduce how reliable it is. A systematic review of four studies found increased breakthrough bleeding in three, reduced hormone exposure in three, and probable ovulation in one when St John's wort was taken with combined oral contraceptives. If you use hormonal contraception, treat St John's wort as a medication change and speak to your prescriber or pharmacist before starting it.
Do I have to avoid vitamin K if I take warfarin?
Consistency matters more than avoidance. In patients on long-term warfarin, higher and steadier dietary vitamin K intake was associated with more stable INR readings, not worse ones. What causes trouble is a sudden change, which is exactly what starting or stopping a K2 supplement is. Do not begin or discontinue one without telling your anticoagulation clinic.
Should I stop fish oil before surgery?
Follow your surgeon's instruction, but the evidence behind that instruction is weaker than most people assume. A randomised trial in 1,516 cardiac surgery patients found no increase in major bleeding with perioperative fish oil and significantly fewer units of blood transfused. The authors called for the stop-before-surgery advice to be reconsidered.
Can I take my iron and calcium at the same time?
Better not to. Calcium reduces non-heme iron absorption, and the effect shows up with calcium from dairy as well as from a tablet. If you are taking iron for a documented low ferritin, separate it from calcium supplements and from dairy-heavy meals. Getting the timing right can be the difference between the course working and not.
Are natural supplements safer to combine with medication than drugs are?
No. An interaction happens because a compound changes how an enzyme or transporter behaves, and plants are very good at that. St John's wort is a plant extract and it is one of the most consequential interacting substances in common use. Natural describes the origin, not the pharmacology.
How do I check my own supplements against my prescriptions?
Put every bottle in a bag, including the ones that seem trivial, and take them to a pharmacist. It costs nothing and takes about ten minutes. They will catch the things a general article cannot: your specific drug, your dose, the timing problem with your thyroid tablet, and the herb hiding in a blend nobody read the back of.
Sources
- Nicolussi S, Drewe J, Butterweck V, Meyer zu Schwabedissen HE. Clinical relevance of St. John's wort drug interactions revisited. Br J Pharmacol, 2020. View study
- Berry-Bibee EN, Kim MJ, Tepper NK, Riley HE, Curtis KM. Co-administration of St. John's wort and hormonal contraceptives: a systematic review. Contraception, 2016. View study
- Bailey DG, Dresser G, Arnold JM. Grapefruit-medication interactions: forbidden fruit or avoidable consequences? CMAJ, 2013. View study
- Kim KH, Choi WS, Lee JH, et al. Relationship between dietary vitamin K intake and the stability of anticoagulation effect in patients taking long-term warfarin. Thromb Haemost, 2010. View study
- Hallberg L, Brune M, Erlandsson M, Sandberg AS, Rossander-Hultén L. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. Am J Clin Nutr, 1991. View study
- Akintoye E, Sethi P, Harris WS, et al. Fish Oil and Perioperative Bleeding. Circ Cardiovasc Qual Outcomes, 2018. View study
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